Alignment Of Rotational Angular Momentum Research Articles

Imaging rotational energy transfer: comparative stereodynamics in CO + N2 and CO + CO inelastic scattering.

State-to-state rotational energy transfer in collisions of ground ro-vibrational state 13CO molecules with N2 molecules has been studied using the crossed molecular beam method under kinematically equivalent conditions used for 13CO + CO rotationally inelastic scattering described in a previously published report (Sun et al., Science, 2020, 369, 307-309). The collisionally excited 13CO molecule products are detected by the same (1 + 1' + 1'') VUV (Vacuum Ultra-Violet) resonance enhanced multiphoton ionization scheme coupled with velocity map ion imaging. We present differential cross sections and scattering angle resolved rotational angular momentum alignment moments extracted from experimentally measured 13CO + N2 scattering images and compare them with theoretical predictions from quasi-classical trajectories (QCT) on a newly calculated 13CO-N2 potential energy surface (PES). Good agreement between experiment and theory is found, which confirms the accuracy of the 13CO-N2 potential energy surface for the 1460 cm-1 collision energy studied by experiment. Experimental results for 13CO + N2 are compared with those for 13CO + CO collisions. The angle-resolved product rotational angular momentum alignment moments for the two scattering systems are very similar, which indicates that the collision induced alignment dynamics observed for both systems are dominated by a hard-shell nature. However, compared to the 13CO + CO measurements, the primary rainbow maximum in the DCSs for 13CO + N2 is peaked consistently at more backward scattering angles and the secondary maximum becomes much less obvious, implying that the 13CO-N2 PES is less anisotropic. In addition, a forward scattering component with high rotational excitation seen for 13CO + CO does not appear for 13CO-N2 in the experiment and is not predicted by QCT theory. Some of these differences in collision dynamics behaviour can be predicted by a comparison between the properties of the PESs for the two systems. More specific behaviour is also predicted from analysis of the dependence on the relative collision geometry of 13CO + N2 trajectories compared to 13CO + CO trajectories, which shows the special 'do-si-do' pathway invoked for 13CO + CO is not effective for 13CO + N2 collisions.

Experimental testing of ab initio potential energy surfaces: Stereodynamics of NO(A2Σ+) + Ne inelastic scattering at multiple collision energies.

We present a crossed molecular beam velocity-map ion imaging study of state-to-state rotational energy transfer of NO(A2Σ+, v = 0, N = 0, j = 0.5) in collisions with Ne atoms. From these measurements, we report differential cross sections and angle-resolved rotational angular momentum alignment moments for product states N' = 3 and 5-10 for collisions at an average energy of 523 cm-1, and N' = 3 and 5-14 for collisions at an average energy of 1309 cm-1, respectively. The experimental results are compared to the results of close-coupled quantum scattering calculations on two literature ab initio potential energy surfaces (PESs) [Pajón-Suárez et al., Chem. Phys. Lett. 429, 389 (2006) and Cybulski and Fernández, J. Phys. Chem. A 116, 7319 (2012)]. The differential cross sections from both experiment and theory show clear rotational rainbow structures at both collision energies, and comparison of the angles observed for the rainbow peaks leads to the conclusion that Cybulski and Fernández PES better represents the NO(A2Σ+)-Ne interaction at the collision energies used here. Sharp, forward scattered (<10°), peaks are observed in the experimental differential cross sections for a wide range of N' at both collision energies, which are not reproduced by theory on either PES. We identify these as L-type rainbows, characteristic of attractive interactions, and consistent with a shallow well in the collinear Ne-N-O geometry, similar to that calculated for the NO(A2Σ+)-Ar surface [Kłos et al., J. Chem. Phys. 129, 244303 (2008)], but absent from both of the NO(A2Σ+)-Ne surfaces tested here. The angle-resolved alignment moments calculated by quantum scattering theory are generally in good agreement with the experimental results, but both experiment and quantum scattering theories are dramatically different to the predictions of a classical rigid-shell, kinematic-apse conservation model. Strong oscillations are resolved in the experimental alignment moments as a function of scattering angle, confirming and extending the preliminary report of this behavior [Steill et al., J. Phys. Chem. A 117, 8163 (2013)]. These oscillations are correlated with structure in the differential cross section, suggesting an interference effect is responsible for their appearance.

Femtosecond Time-Resolved Photofragment Rotational Angular Momentum Alignment in Electronic Predissociation Dynamics.

This Letter presents an experimental and theoretical study of femtosecond time-resolved vector correlations in methyl iodide (CH3I) electronic predissociation via the second absorption B-band at 201.2 nm. The time evolution of the phenomenological anisotropy parameters βl was determined from time-resolved photofragment angular distributions obtained by means of the femtosecond laser pump-probe technique coupled with velocity map imaging detection of vibrational ground-state CH3(ν = 0) fragments and spin-orbit excited I*(2P1/2) atoms. Theoretical interpretation of the experimental results was performed on the basis of a fitting procedure using quasiclassical theory, which elucidates vector correlations in photodissociation of symmetric top molecules. The results of the fitting are in very good agreement with the experimental data and demonstrate the important role of molecular excited-state lifetimes, parent molecule and methyl fragment rotations, and methyl fragment angular momentum alignment on the time-dependent electronic predissociation dynamics.

Rotational alignment effects in NO(X) + Ar inelastic collisions: An experimental study

Rotational angular momentum alignment effects in the rotationally inelastic collisions of NO(X) with Ar have been investigated at a collision energy of 66 meV by means of hexapole electric field initial state selection coupled with velocity-map ion imaging final state detection. The fully quantum state resolved second rank renormalized polarization dependent differential cross sections determined experimentally are reported for a selection of spin-orbit conserving and changing transitions for the first time. The results are compared with the findings of previous theoretical investigations, and in particular with the results of exact quantum mechanical scattering calculations. The agreement between experiment and theory is generally found to be good throughout the entire scattering angle range. The results reveal that the hard shell nature of the interaction potential is predominantly responsible for the rotational alignment of the NO(X) upon collision with Ar.

Rotational alignment effects in NO(X) + Ar inelastic collisions: A theoretical study

Rotational angular momentum alignment effects in the rotational inelastic scattering of NO(X) with Ar have been investigated by means of close-coupled quantum mechanical, quasi-classical trajectory, and Monte Carlo hard shell scattering calculations. It has been shown that the hard shell nature of the interaction potential at a collision energy of Ecoll = 66 meV is primarily responsible for the rotational alignment of the NO(X) molecule after collision. By contrast, the alternating trend in the quantum mechanical parity resolved alignment parameters with change in rotational state Δj reflects differences in the differential cross sections for NO(X) parity conserving and changing collisions, rather than an underlying difference in the collision induced rotational alignment. This suggests that the rotational alignment and the differential cross sections are sensitive to rather different aspects of the scattering dynamics. The applicability of the kinematic apse model has also been tested and found to be in excellent agreement with exact quantum mechanical scattering theory provided the collision energy is in reasonable excess of the well depth of the NO(X)-Ar potential energy surface.

A study of dynamic properties of exchange reaction H(D)+SH/SD by quasi-classical trajectory method

Sulfur in hydrogen combustion reaction chemistry, which plays an important role in meteorology, combustion reactions, and atmospheric pollution, has been extensively investigated recently. And its reverse reaction has also been a research object gradually. The research in this paper is based on the exact potential energy surface (L S J, Zhang P Y, Han K L, He G Z 2012 J. Chem. Phys. 136 094308), with using the method of quasi-classical trajectory on the exchange reaction of H (D)+SH/SD dynamic properties. In this paper, the scalar properties are calculated, including the cross section, rate constant, opacity function, product vibrational, rotational distributions, product scattering direction, rotational angular momentum orientation, and alignment properties. In this paper, how the collision energy and the isotope affect the reaction H (D)+SH/SD kinetic properties is analyzed in detail. The results show that as collision energy increases, the reaction cross section increases, product backscatter weakens gradually while the product rotational angular momentum alignment and orientation nature strengthen gradually. In addition, the isotope effect has a significant influence on the reaction kinetics. The reaction mechanism which is shown in the title and based on the reaction kinetics and the potential energy surface, is also discussed in this paper.

Elastic depolarization and polarization transfer in CN(A2Π, v = 4)+Ar collisions

Rate constants for collisional loss and transfer of population and rotational angular momentum alignment have been determined for the CN(A2Π, v = 4)+Ar system. Aligned samples of CN(A2Π, v = 4, F 1, j = 1.5–23.5e) were prepared by optical pumping on the A–X(4,0) band. Their evolution was observed using Doppler-resolved frequency-modulated spectroscopy in stimulated emission on the A–X(4,2) band. State-resolved total population removal rate constants, and state-to-state rotational energy transfer (RET) rate constants, are found to be in excellent agreement with previous experimental measurements and theoretical predictions for the v = 3 level. Rapid elastic depolarization of rotational alignment was observed for j = 1.5–6.5, with an average rate constant of 1.1 × 10−10 cm3 s−1. This declines with increasing j, reaching zero within experimental error for j = 23.5. The polarization transfer efficiency of the initially created alignment in state-to-state RET was also determined for the selected initial state j = 6.5, F 1, e. Substantial depolarization of the alignment was observed for small Δj transitions. Alignment transfer efficiencies ranged from 0.55 ± 0.06 for Δj = −1, to 0.32 ± 0.08 for Δj = +3. These measurements are discussed with reference to recent experimental and theoretical advances on collisional depolarization of related open-shell species. We suggest that the surprisingly efficient collisional depolarization observed may be the result of the multiple potential energy surfaces involved in this system.

Theoretical Study of the Stereodynamics of CO Collisions with CH3- and CF3-Terminated Alkanethiolate Self-Assembled Monolayers

We present a classical-trajectory study of CO collisions with regular (CH3-terminated) and omega-fluorinated (CF3-terminated) alkanethiol self-assembled monolayers (SAMs) with a focus on analyzing the stereodynamics properties of the collision. The CO molecule is scattered with incident angles of either 30 degrees or 60 degrees with respect to the surface normal and with 60 kJ x mol(-1) collision energy, and we analyze final translational and rotational energy, mechanism of the collisions, and orientation and alignment of the rotational angular momentum. Analysis of the alignment of the final rotational angular momentum in collisions involving initially rotationally cold CO indicates a slight preference for "cartwheel" and "corkscrew" rotational motions. In contrast, collisions of initially excited CO slightly favor "helicopter" motion of the recoiling molecule. Moreover, studies of final orientation reveal that, while cartwheel "topspin" motion is favored for collisions in which initially cold CO becomes rotationally excited, no preferred handedness is observed when CO leaves the surfaces with "helicopter" motion. Analysis of trajectories involving initially rotationally excited CO in which the initial rotational angular momentum is aligned and/or oriented shows a non-negligible effect of the initial rotational motion on the dynamics of energy transfer. For instance, CO approaching the SAMs with helicopter motion retains a larger fraction of its initial rotation than molecules colliding with cartwheel-type motions. Conservation of the alignment and orientation of the initial rotational angular momentum vector is also enhanced with helicopter motion relative to cartwheel or random motions. The calculated trends in the stereodynamic properties for the two SAMs indicate that the CH3-SAM is effectively more corrugated than the CF3-SAM.

Rotational angular momentum polarization: The influence of stray magnetic fields

We show that weak residual magnetic fields can significantly affect the preparation and measurement of molecular rotational angular momentum alignment in a typical gas-phase stereodynamics apparatus. Specifically, polarization spectroscopy, a third-order nonlinear spectroscopic technique, is used to prepare and probe the collisional and noncollisional losses of rotational angular momentum alignment of OH X (2)Pi. Residual magnetic fields of the order of the geomagnetic field are shown to have a significant effect on the prepared polarization on a submicrosecond timescale. This can be expected to be a significant effect for many gas-phase free radicals, such as those of interest in combustion, atmospheric chemistry, and the burgeoning field of cold molecules. We demonstrate a simple experimental remedy for this problem.

Differential scattering cross-sections for CNA2Π+Ar

We present the first results from a novel experimental approach to the measurement of state-to-state differential scattering cross-sections for inelastic scattering of electronically excited CN A(2)Pi with Ar. Photodissociation of ICN with linearly polarized 266 nm radiation generates CN X(2)Sigma(+) (upsilon(")=0,J(")) with a near mono-energetic speed distribution and large anisotropy. Saturated optical pumping of the nascent CN X(2)Sigma(+) transfers this speed distribution without distortion to selected rotational quantum states of the A(2)Pi (upsilon(')=4) level. The products of rotational energy transfer within the A(2)Pi (upsilon(')=4) level into the J(')=0.5, F(2), f, state are probed using frequency modulated stimulated emission spectroscopy on the A-X (4,2) band with a single frequency external cavity tunable diode laser. Doppler profiles of transitions from individual rotational, spin-orbit and lambda doublet specific levels are acquired for different geometrical arrangements of photolysis polarization and probe propagation directions. The resulting Doppler profiles, which for this J(')=0.5 state cannot display a rotational angular momentum alignment, are combined to yield composite Doppler profiles depending on speed and translational anisotropy, which are analyzed to determine fully state-to-state resolved differential scattering cross-sections.

Inelastic scattering of OH(X 2Π) with Ar and He: a combined polarization spectroscopy and quantum scattering study

One-colour polarization spectroscopy (PS) on the OH A (2)Sigma(+)- X (2)Pi(0,0) band has been used to measure the removal of bulk rotational angular momentum alignment of ground-state OH(X (2)Pi) in collisions with He and Ar. Pseudo-first-order PS signal decays at different collider partial pressures were used to determine second-order decay rate constants for the X (2)Pi(3/2), J = 1.5-6.5, e states. The PS signal decay rate constant, k(PS), is sensitive to all processes that remove population and destroy polarization. The contribution to k(PS) from pure (elastic) alignment depolarization within the initial level, k(DEP), can be extracted by subtracting the independently measured or predicted sum of the rate constants for total rotational energy transfer (RET), k(RET), and for Lambda-doublet changing, k(Lambda), collisions from k(PS). Literature values of k(RET) and k(Lambda) are available from experiments with He and Ar, and from quantum scattering calculations for Ar only. We therefore also present the results of new, exact, fully quantum mechanical calculations of k(RET) and k(Lambda) on the most recent ab initio OH(X)-He potential energy surface of Lee et al. [J. Chem. Phys. 2000, 113, 5736]. The results for k(DEP) from this subtraction for He are found to be modest, around 0.4 x 10(-10) cm(3) s(-1), whereas for Ar k(DEP) is found to range between 0.6 +/- 0.2 x 10(-10) cm(3) s(-1) and 1.7 +/- 0.3 x 10(-10) cm(3) s(-1), comparable to total population removal rate constants. The differences between k(DEP) for the two colliders are most likely explained by the presence of a substantially deeper attractive well for Ar than for He. The measurement of k(DEP) may provide a useful new tool that is more sensitive to the form of the long-range part of the intermolecular potential than rotational state-changing collisions.

Role of Rotational Alignment in Dissociative Chemisorption and Oxidation: O2 on Bare and CO‐Precovered Pd(100)

The adsorption of gas-phase molecules that approach solid surfaces in well-defined quantum states represents a fundamental step for the understanding of heterogeneous chemical reactions and for better control over the growth of selfassembled layers. This objective has been achieved only in a few cases to date (NO, H2, [5] CH4 ). Vibrational excitations and conversion of translational to vibrational energy were thereby found to account for strong enhancements in the dissociation probability. Less attention has been devoted to rotations because they involve less energy, and are therefore important only for physisorption. We show here that rotations as well as the alignment of the rotational axis play a role in the dissociative chemisorption of O2 onto a CO Pd(100) surface. The effect, which arises from the requirements needed to pass through the CO adlayer, leads to different sticking probabilities and average O CO distances, and has possible general applications for controlling reactions and film growth. The alignment of rotational angular momentum can be exploited to investigate stereodynamical effects in molecular adsorption processes. 11–13] Such an alignment, which consists of a propensity of molecules to populate specific helicity states (defined by the quantum number M, the projection of the rotational angular momentum along the propagation direction) can be naturally induced by collisions in seeded supersonic molecular beams (MBs). Diatomic, linear, and linear-like molecules flying in the MB with M= 0 behave as cartwheels when impinging on the surface at normal incidence, while when M is maximum they move in a helicopterlike manner. In particular, it has been shown that for the interaction of C2H4 with an O2-precovered Ag(100) surface, 11] and for C3H6/Ag(100), [12] molecules with helicopterlike motion adsorb more efficiently than cartwheeling ones only at intermediate hydrocarbon coverage. The stereoinsensitivity of the initial sticking probability S0 and its coverage dependence indicated that the effect is due to the collision between trapped molecules, which still remember about their original alignment, and preadsorbed ones, which lie flat on the surface. Rotational alignment was found to be ineffective for more strongly bound systems such as s-bonded C2H4/Pd(100) [13] and O2/Ag(100) (interactions mediated via a chemisorbed molecular precursor). This result was attributed to steering forces turning the incoming molecule into the most favorable configuration and causing it to lose memory of its initial state. For O2 on CO-precovered Pd(100), however, we find that molecules moving as cartwheels stick better than those with helicopter-like motion and that molecular alignment also affects the initial sticking probability. The system under investigation is important because of the effective rate of CO oxidation on Pd(100) when exposed to molecular oxygen. Herein we focus on stereodynamical effects of O2 on the S0 value and on the production of CO2. According to Ref. [14], rotational alignment of O2 is generated by collisions during a supersonic seeded expansion of O2 diluted in lighter and faster carrier atoms, typically He and Ne. Under these conditions most of the oxygen molecules in the MB are relaxed in the ground rotational level (K= 1) and cartwheeling motion prevails on average over helicopterlike motion. In addition, as demonstrated by the use of a high-resolution mechanical velocity selector (consisting of eight rotating slotted disks) the angular momentum of the O2 molecules belonging to the slow tail (ST) of the velocity distribution are nearly randomly oriented (ca. 33% inM= 0), while those in the fast tail (FT) fly mostly with cartwheeling motion (> 80%). In the present experiment (see the Experimental Section and the Supporting Information) the fraction moving as cartwheels are approximately 0.4 and 0.7 for the ST and FT regions, respectively. Figure 1 shows the partial pressure of O2 in the chamber, measured by a quadrupole mass spectrometer (QMS), during exposure of the Pd(100) sample at 394 K to the STor FTof the O2 supersonic MB seeded in He at normal incidence. The sample is initially shielded by an inert cover (flag), so that no direct adsorption occurs. The background pressure typically increases by 2 C 10 10 mbar, mostly because of He, so that adsorption from the background over the time scale of an experiment (< 100 s) is also negligible. When the flag is removed (t1) the beam strikes the reactive surface of the sample. The relative decrease in the QMS signal corresponds to the O2 sticking probability S. [16] The lower traces refer to the interaction with bare Pd(100). No difference is observed between the FT and ST experiments, thus indicating either that steering is efficient in turning the molecule into the most favorable configuration for dissociation or that the interaction [*] Dr. A. Gerbi, Dr. L. Savio, Dr. L. Vattuone Dipartimento di Fisica CNISM Unit) di Genova Universit) di Genova Via Dodecaneso 33, 16146 Genova (Italy) Fax: (+39)010-314-218 E-mail: vattuone@fisica.unige.it Homepage: http://www.fisica.unige.it/~ vattuone

Orientation and alignment moments in two-color polarization spectroscopy

A theoretical analysis of two-color polarization spectroscopy (TCPS) is presented as an extension of a previous analysis of one-color PS. Three commonly used schemes in which pump and probe transitions share a common level are considered. Diagrammatic techniques are used to isolate the photon interaction sequences that can contribute to the signal. A perturbation-theory analysis expressing the signal in terms of spherical tensor moments is applied. The analysis emphasises the significance of orientation and alignment tensor moments of rotational angular momentum and their collisional evolution. The assumed context is transitions between single rotational states of gas-phase molecules that subsequently suffer discrete collisions. The time scale of the measurements is assumed to be long relative to the periods of molecular motion, as would typically be the case for signals excited by nanosecond-pulsed lasers from samples at moderate pressures. The Doppler motion of the probed species is included, as is an analytical solution to the integration over the Maxwell-Boltzmann distribution of velocities. The effects of nuclear hyperfine depolarization and velocity-changing collisions are discussed. It is shown that when pump- and probe-laser pulses are separated in time, TCPS creates and probes either orientation or alignment of rotational angular momentum in the common level shared by pump and probe transitions. Example simulations of one- and two-color polarization spectroscopies are included to demonstrate the resulting simplification of the measured signal using TCPS. TCPS is therefore a viable spectroscopic technique for the determination of rotational angular momentum orientation and alignment relaxation rates in molecular gases, of interest because they are sensitive probes of inelastic collisions.

Insertion and abstraction pathways in the reaction O(1D2) + H2-->OH+H.

Rigorous quantum dynamical calculations have been performed on the ground 1 1A' and first excited 1 1A" electronic states of the title reaction, employing the most accurate potential energy surfaces available. Product rovibrational quantum state populations and rotational angular momentum alignment parameters are reported, and are compared with new experimental, and quasiclassical trajectory calculated results. The quantum calculations agree quantitatively with experiment, and reveal unequivocally that the 1 1A" excited state participates in the reaction.

Reaction cross-section and product polarization in the reaction

Abstract The Ca ( 1 D 2 ) + HBr → CaBr(A,B) + H reaction has been studied by measuring its chemiluminescence at an average collision energy of ET = 0.16 eV under beam-gas conditions. Absolute values of the reaction cross-section for both chemiluminescence channels (e.g., A and B electronic states) were found to be σ A = 2.2 ± 0.9 A 2 and σ B = 0.52 ± 0.24 A 2 . In addition, the product polarization of the chemiluminescence emission was also measured, indicating a strong product rotational angular momentum alignment. The results are discussed in the light of a recent stereodynamical model for these, heavy + heavy-light, kinematically constrained reactions.

Alignment of CN from 248 nm photolysis of ICN: A new model of the à continuum dissociation dynamics

A KrF excimer laser (248 nm) is used to dissociate a low pressure (5–10 mTorr) sample of cyanogen iodide (ICN) and the resulting CN X 2Σ+ fragments are probed by laser-induced fluorescence (LIF) via various vibrational sequences of the B 2Σ+–X 2Σ+ transition. In addition to measuring relative vibrational and rotational populations in the CN X 2Σ+ photofragment, the alignment of rotational angular momentum in this fragment is determined from the variation in LIF intensity as the direction of linear polarization of the probe laser is rotated with respect to that of the photolysis laser. A unifying model is proposed for the à continuum photodissociation dynamics which is able to account for present and previous experimental measurements characterizing both the I and CN photofragments.

Measurement of product rotational alignment in associative-ionization collisions between polarized Na(3p) atoms.

We have studied the effect of reactant Na(3p) polarization on the rotational angular momentum alignment of product ${\mathrm{Na}}_{2}$${\mathrm{}}^{+}$ ions arising from associative-ionization (AI) collisions. Our results show that sensitivity of the AI rate constant to initial atomic polarization persists even when all hyperfine states are populated with broadband (3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) pulsed laser excitation of Na${(\mathrm{}}^{2}$${P}_{3/2}$) and that the spatial distribution of product rotational angular momentum vectors is anisotropic. We discuss a qualitative description of the collision process consistent with our measurements which indicates that \ensuremath{\sigma}-orbital symmetry is preferred to \ensuremath{\pi}-orbital symmetry as the colliding partners approach.

Flow birefringence in gases of linear and symmetric top molecules

Abstract Gradients in the stream velocity of a gas consisting of optically anisotropic molecules give rise to an alignment of rotational angular momenta. This alignment can be observed as a birefringence in the gas. The results of such measurements are reported in this paper and interpreted within the framework of a kinetic theory based on the Waldmann-Snider equation. A comparison is made with data obtained from other experiments such as the Senftleben-Beenakker effect and the depolarized Rayleigh scattering.